1. Field of the Invention
The present invention relates to a method for preparing polypropylene terephthalate/polyethylene terephthalate copolyester (referred to as PPT/PET copolyester hereinafter), and more particularly the invention relates to a method for preparing PPT/PET copolyester by employing a process selected from between the process (a) and the process (e), whereas: process (a) involves reacting BHET, PTA and 1,3-PDO first by an esterification reaction and then by a copolymerization reaction; process (b) involves subjecting PTA and 1,3-PDO in undergoing an esterification reaction, adding BHET to the content, and then reacting the content by a copolymerization reaction; process (c) involves directly copolymerizing BHET and BHPT; process (d) involves reacting PTA, ethylene glycol (EG) and 1,3-PDO first by an esterification reaction and then by a copolymerization reaction; and process (e) involves reacting BHPT, PTA and EG first by an esterification reaction and then by a copolymerization reaction.
2. Description of the Prior Art
Generally speaking, polyethylene terephthalate (PET) is a material that has high strength and high modulus, but it is difficult to be dyed in rich colors. On the other hand, polypropylene terephthalate (PPT) is a material that can be easily dyed and has high elastic resilience, but its disadvantages are low strength and low melting point.
Therefore, many efforts have been made to improve on the dyeability of a fiber grade PET. As disclosed in U.S. Pat. No. 4,049,633 (1977), U.S. Pat. No. 4,029,637 (1977), U.S. Pat. No. 4,029,638 (1977), and U.S. Pat. No. 4,668,764 (1987), a dyeing auxiliary agent is added during the polymerization reaction to obtain a copolyester. The dyeing auxiliary agents commonly used include N,N,N',N'-tetramethyl-1,8-diaminonaphthalene, N-(.beta.-hydroxyethyl)-N-(3-carbomethoxybenzesulfonyl)taurine, alkali metal salt of N-benzyl-N-propyl sulfonate benzene sulfonamide, 1,4-cyclohexanedimethanol, and 2,2-bis-propane. As mentioned above, the dyeability of the copolyester can be improved by the addition of the dyeing auxiliary agent, however, the processability will be decreased such that the whole process is adversely affected. For example, not only does the commercial cationic dyeable PET in fiber form has a lower strength than conventional polyester fiber, but it also has a shorter pack life during melt spinning than conventional polyester. In addition, the cationic dyeable PET has the disadvantage of not being able to be produced in continuous mode.
Thus, researchers have thought of copolymerizing PPT and PET together to form a new polyester with improved physical properties. Ponnusamy and Balakrishnan develop a PET/PPT copolyester, which is produced by reacting dimethyl terephthalate (DMT), ethylene glycol, and 1,3-propanediol in a melt-polycondensation reaction (J. Macromol. Sci.-Chem., A22 (3), pp. 373-378 (1985)). Nonetheless, methanol by-product is difficult to be recovered while DMT has always been a very expensive compound. In addition, the PET/PPT copolyester obtained through the melt-polycondensation reaction has too small a molecular weight, and the maximum intrinsic viscosity is only 0.4 dL/g with no practical value.
On the other hand, Yang Ho Park et al. develop another PET/PPT copolyester, which is produced by reacting PET oligomer and 1,3-propanediol (1,3-PDO, also abbreviated as PG) in a polycondensation reaction (Journal of the Korean Fiber Society, Vol. 36, No. 7, 1999). The PET oligomer (BHET) reacts only with the end residual groups of the PG compound. Therefore, theoretically, the bis-2-hydroxypropyl terephthalate (BHPT) chain obtained in the copolyester product is very limited. In addition, the reaction temperature is between 240.degree. C. and 280.degree. C., which exceeds the boiling point of PG. In addition, the alcohol of the PET oligomer should be reacted with 1,3-propanediol by an interchange reaction and then by the polycondensation reaction to produce the PPT/PET copolyester. The ethylene glycol by-product formed during the interchange reaction must be removed, thus the production cost is adversely increased.